1. Field of the Invention
The present invention relates to the prevention of crackings in continuously cast carbon or alloy steel slabs containing boron.
2. Description of the Prior Art
In recent years, in the fields of low carbon steels used for automobile car bodies and for production of galvanized sheets, and medium carbon steels used for production of high tension steel plates for machine structural use, and further in the fields of high carbon steels, etc. for production of bar rods or seamless pipe, more and more boron containing steels have been used.
Boron, when added in a very small amount in steels, markedly improves the quality (particularly strength-ductility balance) of thin steel sheet products, or in the cases of high tension steel plates or steel bars, considerably improves the martensite hardenability, so that it is possible to reduce the carbon content in the steels and to improve the weldability of the steels. Further, boron can substitute precious alloying elements, such as nickel and chromium, in wide applications, so that it is possible to lower the alloying cost. Therefore, the addition of boron in steels is very advantageous for improvements of the steel quality and reduction of the production cost.
As well known, these grades of boron-containing steels are conventionally processed into ingots by the ingot-making process and the resultant ingots are subjected to hot rolling and heat treatment processes into final products.
In the ingot-making process, the molten steel in the ingot mold solidifies over a long period of time so that a satisfactory surface quality of the ingot can be assured and the surface defects of the ingot as discussed hereinafter have caused no substantial problem.
In recent years, for saving the energy, and for improving the productivity and lowering the production cost by simplifying the production processes, more and more boron-containing steels have been processed into steel slabs by the continuous casting process in substitution of the ingot-making process. However, when boron-containing steel slabs are produced by an ordinary continuous casting machine, many defects are caused not only in the internal quality, but also in the surface quality. This problem has been prohibiting a commercial production of boron-containing steel slabs by continuous casting.
To illustrate examples, when about 10 to 30 ppm boron is added to low-carbon Al-killed molten steels or Al-Si killed molten steels which are presently cast by continuous casting, the resultant continuously cast slabs have often a remarkably deteriorated surface quality. And once such surface defects have been caused on boron-containing steel slabs, hot or cold stage scarfing either by gas or mechanical surface grinding must be performed on the partial or whole surface of the slabs to remove the defects, and this scarfing not only requires much labour and cost, but also markedly lowers the production yield of the slabs, thus killing the advantages inherent to the continuous casting process.